Methane is a valuable energy resource, but, after carbon dioxide, is the largest contributor to global warming. Methane accounts for about 10% of U.S. greenhouse gas emissions, and has almost eight times the global warming effect of an equivalent volume of carbon dioxide (21 times on a mass basis). Most emissions take the form of dilute mixtures of methane and carbon dioxide, with a methane content below 70%, and arise from landfills, from natural gas and oil processing, and from cattle or other biogas sources.
Many landfill, biogas and natural gas processing vent streams, for example, contain 20 to 70% methane. These streams have a low Btu value (200 to 700 Btu/Scf), too low to be used as fuel in conventional engines, which generally require a minimum of 700 Btu/Scf. To dispose of these gases, they are either vented or mixed with supplemental fuel to bring the gas up to a Btu value of 500 Btu/Scf, at which point the gas can be flared. In either case, the heating value of the methane in the gas is lost.
If the concentration of methane in the stream could be increased, it would be possible to use the stream as combustion fuel, or at least to dispose of it by flaring, thereby converting the methane to carbon dioxide before it is emitted. However, such streams are difficult to treat in ways that are both technically and economically practical, and there remains a need for better treatment techniques.
Gas separation by means of membranes is a well established technology. In an industrial setting, a total pressure difference is usually applied between the feed and permeate sides, typically by compressing the feed stream or maintaining the permeate side of the membrane under partial vacuum.
It is known in the literature that a driving force for transmembrane permeation may be supplied by passing a sweep gas across the permeate side of the membranes, thereby lowering the partial pressure of a desired permeant on that side to a level below its partial pressure on the feed side. In this case, the total pressure on both sides of the membrane may be the same, or there may be additional driving force provided by keeping the total feed pressure higher than the total permeate pressure.
Using a sweep gas has most commonly been proposed in connection with air separation to make nitrogen or oxygen-enriched air, or with dehydration. Examples of patents that teach the use of a sweep gas on the permeate side to facilitate air separation include U.S. Pat. Nos. 5,240,471; 5,500,036; and 6,478,852. Examples of patents that teach the use of a sweep gas in a dehydration process include U.S. Pat. Nos. 4,931,070; 4,981,498 and 5,641,337.
Configuring the flow path within the membrane module so that the feed gas and sweep stream flow, as far as possible, countercurrent to each other is also known, and taught, for example in U.S. Pat. Nos. 5,681,433 and 5,843,209.